17.Evolution of sleep
Sleep is very ancient. In the electroencephalographic sense we share it with all the primates1 and almost all the other mammals and birds: it may extend back as far as the reptiles2. There is some evidence that the two types of sleep, dreaming and dreamless, depend on the life-style of the animal, and that predators3 are statistically4 much more likely to dream than prey5, which are in turn much more likely to experience dreamless sleep. In dream sleep, the animal is powerfully immobilized and remarkably6 unresponsive to external stimuli7. Dreamless sleep is much shallower, and we have all witnessed cats or dogs cocking their ears to a sound when apparently8 fast asleep. The fact that deep dream sleep is rare among pray today seems clearly to be a product of natural selection, and it makes sense that today, when sleep is highly evolved, the stupid animals are less frequently immobilized by deep sleep than the smart ones. But why should they sleep deeply at all? Why should a state of such deep immobilization ever have evolved? Perhaps one useful hint about the original function of sleep is to be found in the fact that dolphins and whales and aquatic9 mammals in genera seem to sleep very little. There is, by and large, no place to hide in the ocean. Could it be that, rather than increasing an animal’s vulnerability, the University of Florida and Ray Meddis of London University have suggested this to be the case. It is conceivable that animals who are too stupid to be quite on their own initiative are, during periods of high risk, immobilized by the implacable arm of sleep. The point seems particularly clear for the young of predatory animals. This is an interesting notion and probably at least partly true.
1 primates
primate的复数
参考例句：
Primates are alert, inquisitive animals. 灵长目动物是机灵、好奇的动物。
Consciousness or cerebration has been said to have emerged in the evolution of higher primates. 据说意识或思考在较高级灵长类的进化中已出现。
2 reptiles
n.爬行动物，爬虫( reptile的名词复数 )
参考例句：
Snakes and crocodiles are both reptiles. 蛇和鳄鱼都是爬行动物。 来自《简明英汉词典》
Birds, reptiles and insects come from eggs. 鸟类、爬虫及昆虫是卵生的。 来自《现代汉英综合大词典》
3 predators
n.食肉动物( predator的名词复数 )；奴役他人者（尤指在财务或性关系方面）
参考例句：
birds and their earthbound predators 鸟和地面上捕食它们的动物
The eyes of predators are highly sensitive to the slightest movement. 捕食性动物的眼睛能感觉到最细小的动静。 来自《简明英汉词典》
4 statistically
ad.根据统计数据来看，从统计学的观点来看
参考例句：
The sample of building permits is larger and therefore, statistically satisfying. 建筑许可数的样本比较大，所以统计数据更令人满意。
The results of each test would have to be statistically independent. 每次试验的结果在统计上必须是独立的。
5 prey
n.被掠食者，牺牲者，掠食；v.捕食，掠夺，折磨
参考例句：
Stronger animals prey on weaker ones.弱肉强食。
The lion was hunting for its prey.狮子在寻找猎物。
6 remarkably
ad.不同寻常地，相当地
参考例句：
I thought she was remarkably restrained in the circumstances. 我认为她在那种情况下非常克制。
He made a remarkably swift recovery. 他康复得相当快。
7 stimuli
n.刺激（物）
参考例句：
It is necessary to curtail or alter normally coexisting stimuli.必需消除或改变正常时并存的刺激。
My sweat glands also respond to emotional stimuli.我的汗腺对情绪刺激也能产生反应。
8 apparently
adv.显然地；表面上，似乎
参考例句：
An apparently blind alley leads suddenly into an open space.山穷水尽，豁然开朗。
He was apparently much surprised at the news.他对那个消息显然感到十分惊异。
9 aquatic
adj.水生的，水栖的
参考例句：
Aquatic sports include swimming and rowing.水上运动包括游泳和划船。
We visited an aquatic city in Italy.我们在意大利访问过一个水上城市。
